docs: def serve_once(self):
          """ Listen and handle 1 request. """
          # 256 is the maximum size of a Modbus RTU frame.
          request_adu = self.serial_port.read(256)
          log.debug('<-- {0}'.format(hexlify(request_adu)))
  
          if len(request_adu) == 0:
              raise ValueError
  
          response_adu = self.process(request_adu)
          self.respond(response_adu)